The designers of steam turbines seek for quick selection of useful blades with a minimum number of inventory. One would prefer a few efficient blades to cover a wide flow range prevailing in turbine stages. There are publications such as Deich et al. (Atlas of Blades Profiles for Axial Turbines 1965) for a set of profiles. Further, two patents U.S. Pat. No. 5,211,703 (1993) and U.S. Pat. No. 5,192,190 (1993) on stationary blade have been filed by the authors, viz. Ferteger, Jurek and Evans, David H. Such patents were for a twisted stationary blade with varying stagger angle from hub to tip (from 42 deg at hub to 52 deg at shroud). The blade is non-cylindrical and twisted over the span. A recent patent by the present author (U.S. Pat. No. 6,709,239) is for design of three dimensional twisted blade for use in entry stages of HP/IP cylinders of axial steam turbines. A related patent by Purcaru et al. (U.S. Pat. No. 4.695.228) deals with the construction of profiles through ellipse, parabola and circle segments. The present author has also filed an application (Pub. No. U.S. 2003/0231961A1, U.S. Pat. No. 6,979,178B2) for two cylindrical profiles for subsonic flow application and for a specified range of stagger angles. One of the profiles, P2822 is the reference profile for the present invention which concerns with a new blade profile; that can be used for forming a cylindrical blade i.e. with constant stagger from hub to tip. The blades formed by this profile are untwisted or cylindrical in shape. In addition, the present invention deals with both stationary (guide) and rotating (moving) type of blades for axial steam turbines.
While converting heat energy into kinetic energy, turbines blades suffer two kinds of aerodynamic losses; one—the profile loss due to stream wise boundary layer growth (along blade surfaces), and, mixing in blade wakes, the second—the profile loss due to secondary flow resulting from boundary layer growth along the hub and casing and flows resulting from turning of inlet boundary layer (passage vortex; pressure face to suction face in a cascade passage). The reduction in losses is achieved by various means such as smooth surface and aft-loaded pressure distribution along the blade surfaces (instead of fore-loaded or flat-topped design). Smooth contour variation usually ensures lower profile losses for incompressible and subsonic flows. The lower velocity and cross-channel pressure gradient in the first part of cascade passage where the secondary flow originates; and higher diffusion in the rear part of suction face are the desired features in aft-loaded profiles which in turn reduces secondary flow losses.
The cylindrical blade is defined herein as one of constant cross-section over the blade height. FIG. 1 shows a schematic base profile. At any cross-section, the shape of the profile remains same as shown typically in FIG. 2. The profile or section is made of two surfaces; suction face and pressure face, each joining leading edge to trailing edge. X-axis and U-axis coincide with the turbine axis and circumferential directions, respectively. Usually the center of gravity lies at the origin of co-ordinate axes. The blade or profile is set at angle betabi or y, tg, also known as stagger or setting angle with respect of U-axis. Chord is defined as axial distance of base profile measured between two farthest tangents to the profile; one at leading edge side and other at trailing edge side. The tangents are normal to the chord. Axial chord is the projected length of the profile on X-axis; hence varies with profile stagger. Inlet and exit flow angles β1, tg and β2, tg are fluid flow angles with respect to tangent (U-axis); also referred as beta 1x and beta 2x with reference to turbine axis, respectively. The profile faces can be specified by various ways; e.g. through discrete points (x, y co-ordinates), through a set of arcs and through Bezier points. The basic difference between any two cylindrical blades is the profile shape and what is being claimed here is the unique quantitative shape of the proposed blade (e.g. geometrical ratios as shown in FIG. 3).